The process of excitation-contraction coupling involves a sequence of poorly understood steps leading from fiber depolarization to contractile activity. We propose to quantitatively investigate several of the intermediate steps in order either to determine the kinetic relationships between supposed successive steps or, alternatively, to establish that the events are not causally related. A new cut single muscle fiber preparation which permits modification of internal composition and provides for voltage-clamp control of membrane potential with current recording during contraction will be used. The following processes will be monitored: (1) Intra-membrane charge movements thought to be the initial voltage-sensitive step; (2) Optical signals which may be due to changes in SR membrane potential related to calcium release; (3) Myoplasmic free calcium concentration transients; (4) Fiber shortening and/or tension development.